The present invention refers to a bleed air heating system for an auxiliary power unit (APU) compartment of an aircraft, in order to de-ice the fuel trapped in the APU before starting and after being disconnected for a long time under freezing conditions.
The auxiliary power unit (APU) is a gas turbine engine that traditionally supplies electric and pneumatic power to the aircraft systems as an auxiliary or secondary source of power. The APU allows the aircraft to be autonomous of external electric and pneumatic power sources on ground and in-flight.
This electric and pneumatic power can be provided to the aircraft separately or in combination. The pneumatic power provides compressed air for the cabin and power for main engine starting.
FIG. 1 shows a conventional APU bleed supply architecture. As shown, the architecture comprises an APU (3) held in the APU compartment (2) provided in the tail cone of the aircraft (1), an APU bleed valve (4) configured to open and close the bleed air passage from the APU (3), and an APU bleed air duct (5) for conveying the APU bleed air towards the main engines (6).
Some bleed supply architectures further comprise an isolation valve (11) to protect the APU (3) when no compressed air is being supplied.
Under certain conditions, the aircraft's fuel system may provide fuel with a high concentration of liquid water. Once the APU is stopped, this mixture of fuel and water gets trapped inside all the fuel system cavities, conducts, valves, pumps, actuators, sensors, etc.
It frequently occurs that the APU gets exposed to water subfreezing temperatures during long periods of time, such as when aircraft is parked overnight or during cruise flight, where APU shuts off once the main engines get started.
In these cases, lumps of liquid water get solidified and cause internal fuel system blockages that impede the start and the normal running process of the APU.
It would therefore be desirable to provide technical means to an aircraft for defrosting the fuel trapped in the APU, to thus improve the starting and operation phases of the APU irrespective of the freezing conditions.